1. Field of the Invention
The present invention relates generally to cardiac pacemakers, and more particularly to cardiac pacemakers having a markers channel corresponding to and indicative of pacing events of the pacemaker.
2. Background of the Prior Art
During unipolar and bipolar pacing, the pacing spike may be hard to ascertain in the surface EKG. Thus, the resulting electrocardiogram many times does not permit a physician the ability to interpret the behavior of the implanted pacemaker to evaluate the adequacy of the therapy that is being delivered to the patient. At high pacing rates the analysis is even more difficult, and the task of distinguishing between atrial paced or sensed beats becomes sometimes formidable. Another problem is to know why the pacemaker is delivering a given therapy. Situations like "fall-back", "rate smoothing", "sensor rate", "sensing during refractory", "upper rate limit behavior", etc. are also very difficult to interpret from surface EKG.
Prior art pacemakers typically incorporate telemetered marker channels to help interpret pacemaker behavior, which marker channels include coding generated concurrently by the pacemaker during pacing. Presently, the marker channel is sensed using well-defined external devices in combination with telemetry functions built into the pacemaker. The marker channel provides coded information indicative of the pacemaker therapy currently being applied to the heart such that the attending physician can intelligently compare the ascertained EKG to the pacing therapy being applied by the pacemaker.
The ERGOS 03 pacemaker offered by Biotronik provides the ability to use a marker channel by sending below-stimulation threshold pulses to the endocardial electrodes immediately after pacing, which pulses can be recognized on a surface EKG. Atrial sensed events are marked by a 30 .mu.S pulse that is emitted by the atrial lead. Ventricular sensed events are marked by two sequential marker signals emitted by the ventricular lead. The two ventricular pulses have a spacing of 60 milliseconds, the first denoting the moment of the sensed event. This particular system has two main disadvantages. First, during committed pacing situations issuing a sub-threshold pacing pulse to the ventricular or atrial leads, a transitory conduction block can occur. Issuing a supra-threshold pacing pulse immediately thereafter could cause arrhythmias. Secondly, this scheme only allows the pacemaker to send markers to the surface EKG during sensing, and does not provide enough resolution or time to code in several markers to indicate situations like atrial sensed during PVARP, pacing due to rate smoothing, ventricular ectopic sensing, etc. Moreover, this scheme does not allow the pacemaker to send markers simultaneously while pacing the heart. Finally, this scheme does not allow generating supra-threshold markers.
Several patents issued to Medtronic teach pacemakers incorporating marker channels using well-known telemetry functions. However, these devices require telemetry equipment and receivers, and diagnosis needs to be performed in the presence of an attending physician with the appropriate equipment and telemetry receivers. Thus, patients who feel irregularities in their pacemaker operation only occasionally and intermittently at home cannot be properly diagnosed by a physician for adjustments to their pacemakers. Prior art pacemakers having marker functions are taught in U.S. Pat. No. 4,550,370 and U.S. Pat. No. 4,548,209 to Medtronic. Each of the devices taught implements telemetry for transmission of programming codes and to receive marker information from the pacemaker for remote display and utilization. Similarly, U.S. Pat. No. 4,595,009 and U.S. Pat. No. 4,374,382 also to Medtronic teach marker channel telemetry systems for medical devices, as do U.S. Pat. Nos. 4,601,291 and 4,559,947.
To help understand the behavior of a pacemaker which may be operating in an unknown manner, the physician will provide the patient with a multi-channel holter monitor to sense and store EKG signals over an extended time period, such as 24 hours, while the patient is at home and away from the physician's office. Unfortunately, prior art diagnostic methods using holter monitors only sense and store EKG signals from one or more locations of a patient's body, which EKG signals are subsequently studied by the physician. Consequently, the physician's ability to diagnose the pacemaker behavior is limited to study of the recorded EKG signals since telemetry of the marker channels cannot be ascertained to help a physician understand the pacemakers behavior.